The present invention relates to an electronically controllable brake booster. The invention relates in particular to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from one another by a moveable wall. A control valve arrangement can be actuated by means of an electromagnetic actuating device. A pressure difference between the pressure and the vacuum chamber can be set via the control valve arrangement. The control valve arrangement takes up a holding position, a pressure buildup position or a pressure reduction position in accordance with a current generated as correcting variable by an electronic control unit and flowing through the electromagnetic actuating device. A cylinder-piston arrangement is connected to the brake booster, this arrangement comprising a piston, coupled to a brake pedal, of a hydraulic cylinder which has a hydraulic connection leading to a wheel brake. A sensor arrangement serves to detect a signal which is correlated with the pressure of the hydraulic cylinder and which reproduces the controlled variable. A brake booster of this kind is described, for example, in DE 19527 493 A1, which is incorporated by reference herein, together with a learning process for the operating point setting.
The brake booster illustrated above is operated in a closed loop. This ensures that the electromagnetic actuating device is largely prevented from overshooting or undershooting. The control characteristic is subject to a variation range in the region of the two sloping branches due to interference effects caused, for example, by friction losses upon the armature and the valve components coupled thereto moving, or by tolerances of the spring arrangement, or by fluctuations of external reaction forces, which include in particular fluctuations in the compressive force component in the vacuum chamber when using a brake booster. This proves to be particularly problematic for an accurate setting of the xe2x80x9cholding positionxe2x80x9d. If, for example, a change-over from xe2x80x9cbuild-upxe2x80x9d position to xe2x80x9cholding positionxe2x80x9d is desired, it is possible for a change-over to occur immediately after xe2x80x9creduction positionxe2x80x9d, which results in an abrupt pressure drop which may have serious consequences, especially where safety-critical applications are concerned, as in the case of an electronically controlled brake booster. In order to counter this problem, according to-the teaching of this prior art, when changing over to xe2x80x9cholding positionxe2x80x9d a current I0 resulting as the arithmetic average value of the currents I1 and I2 is always set, as the arrangement is designed such that the lower limit of the variation range of the left-hand sloping branch and the upper limit of the variation range of the right-hand sloping branch do not overlap.
The present invention takes up the approach to a solution from DE 195 27 493 A1 and directly includes the driver""s braking requirement in this concept. However the consideration of the pressure which is described in this prior art does not determine the object.
In order to solve this problem in a brake booster of the type initially described, a command variable is established by the electronic control unit, taking account of the output signal of a sensor arrangement detecting the driver""s braking requirement, wherein the value of the correcting variable remains unchanged with respect to a value output directly beforehand if the control difference established from the command variable and the controlled variable does not deviate from a first predetermined tolerance range value.
The result of this measure is to xe2x80x9csmoothxe2x80x9d the actuation of the actuator without causing any impairment of the control quality.
In a preferred embodiment the value of the correcting variable remains unchanged with respect to a value output directly beforehand if a (the first or second) derivative of the control difference also does not deviate from a second predetermined tolerance range value. The control behaviour is also adapted to the dynamic behaviour of the overall arrangement by including the gradients of the control deviation.
Instead of using the derivative of the control difference, the value of the correcting variable may remain unchanged with respect to a value output directly beforehand if an intermediate variable established from a derivative of the command variable and a derivative of the controlled variable also does not deviate from a second predetermined tolerance range value.
In a preferred embodiment of the invention the control valve arrangement can be brought by a first current value of the correcting variable into the pressure build-up position, in which the connection between the vacuum chamber and the pressure chamber is blocked and the connection between the pressure chamber and the atmosphere is open, so that a pressure difference is built up or increased at the moveable wall, by a second current value of the correcting variable into the pressure holding position, in which the connection between the vacuum chamber and the pressure chamber is blocked and the connection between the pressure chamber and the atmosphere is blocked, so that a pressure difference acting at the moveable wall is maintained, or by a third current value of the correcting variable into the pressure reduction position, in which the connection between the vacuum chamber and the pressure chamber is open and the connection between the pressure chamber and the atmosphere is blocked, so that a pressure difference acting at the moveable wall is reduced by a pressure equalisation procedure.
According to the invention, the correcting variable assumes the first current value if the command variable is greater than the controlled variable, the second current value if the command variable is substantially equal to the controlled variable, and the third current value if the command variable is less than the controlled variable.
In a preferred embodiment of the invention an additional proportional and/or integral and/or differential controller serves to convert the control difference into an auxiliary correcting variable so as to superimpose this on the correcting variable, preferably in additive fashion
In a development a linear transmission member with a dead zone may be connected upstream of the additional proportional and/or integral and/or differential controller.